The present application relates to ballast means and more particularly to a capacitively ballasted circuit providing low voltage to an incandescent lamp.
As is well known in the art, incandescent lamps operating at a nominal voltage of approximately 120 volts do not provide as high an efficacy, that is lumens per watt, at the same wattage as incandescent lamps operating at lower voltage levels. Prior art circuitry providing such lower voltage operation either exhibits undesirable cost, volume, weight or high levels of electromagnetic interference. In particular, some prior art low voltage power supplies for incandescent lamps have utilized magnetic components for voltage transformation. The cost of such magnetic components has prevented the resulting power supplies from being economically attractive. Other power supplies have utilized phase-control techniques, in which very narrow pulses are required, with high surge currents flowing through the load; frequently resulting in electromagnetic interference and reduced reliability.
As disclosed in cross-referenced application 379,412, a capacitively ballasted incandescent lamp comprising a main capacitor in series with an incandescent lamp load and one or more auxiliary capacitors, electrically connected in parallel with the main capacitor, avoids the problems described heretofore. However the switching means, such as a triac, disclosed therein, which switch each additional capacitor electrically in parallel with the main capacitor, has several disadvantages including a higher cost and lower maximum operating temperature than is desired.